What device is used to prevent backflow by allowing air to be admitted to the supply pipe during a vacuum?

The device used to prevent backflow by allowing air to enter the supply pipe during a vacuum is the vacuum breaker. This mechanism is designed to avert the conditions that lead to back siphonage, which can occur when there's a sudden drop in water pressure. When a vacuum forms in the plumbing system, the vacuum breaker automatically opens to allow air to enter the supply line, breaking the vacuum and ensuring that contaminated water cannot be pulled back into the potable water supply. This is essential for maintaining the integrity and safety of drinking water systems.

In contrast, other options serve different purposes. A gate valve is primarily used for isolating sections of piping and does not provide backflow prevention. An air relief valve is designed to release trapped air in a pipeline but does not function as a vacuum breaker. A float valve is used to control the level of water in tanks and cisterns but does not specifically address vacuum situations.